Abstract Exciting pre-clinical research supports a role for gut microbes (microbiomes) in modulating brain function and behavior. An important observation stemming from studies in animals is that developing brains are more susceptible to the effects of microbiome modulation than mature brains, supporting the idea that there is an early-life sensitive period for brain developmental programming by the microbiome. The hippocampus, a brain region responsible for learning and memory, is a target of microbiome-mediated effects in adult mice; however a relationship between early-life microbiomes and programming of hippocampal function in humans has not been studied. In preliminary experiments, we have found that an infant cohort predicted to have disrupted gut microbiomes (due to antibiotic exposure) exhibited abnormal recognition memory responses (indexed by event-related potentials (ERPs)) at 1 month of age, as compared to unexposed infants. Given these findings, we hypothesize that variations in microbiome signatures during infancy correlate with differences in hippocampus function, and that early-life events that disrupt the microbiome will result in a developmental delay in acquisition of learning and memory functions in affected infants. In the proposed studies, we capitalize on our group?s unique abilities to quantify hippocampal function in very young infants and to compare these functions to microbiome features using state-of-the-art computational strategies to discover microbiome determinants of hippocampus function. In Aim 1, we will use natural variation in microbiome composition in a healthy infant cohort that is relatively free of confounding variables to define microbiome groups. In Aim 2, we will compare two groups of otherwise healthy infants that differ with respect to microbiome disruption (antibiotic exposure) as newborns. For both Aims, we will characterize and compare human hippocampus function during its most rapid phase of development, in very young infant microbiome cohorts, using cutting-edge infant neurobehavioral testing approaches that our team at the U of MN Center for Neurobehavioral Development is uniquely qualified to execute. Infants will be compared using established auditory (1 mo of age) and visual (6 mos of age) recognition ERP paradigms to determine if microbial variation modulates hippocampus function and, further, to identify specific healthy microbiome features (biodiversity, abundances of key taxa) that define optimum hippocampus function. The potential high-impact gain of this R21 research is that it will provide evidence that gut microbes affect the brain at very early times in postnatal human neurodevelopment with a likelihood, then, of shaping long-term brain health. In contrast to genetic factors, gut microbes can be modified. Thus, our results will lay the foundation for future studies to develop microbiomes for early interventions and for protection of healthy microbiomes via improved antibiotic stewardship, during the developmental window of high brain plasticity, to improve brain development in at-risk individuals.